Magnesium, as a metal having a specific gravity of 1.74, is being in the spotlight as an aerospace material and an exterior material of electronic devices, because it is not only the lightest metal among metallic materials, but it also has excellent specific strength, dimensional stability, electromagnetic shielding, and heat dissipation properties. However, since general characteristics of magnesium in terms of strength and corrosion are not suitable for being used as a structural material, magnesium is being used in the form of alloys to which various elements are added.
Since most of the magnesium alloys having a hexagonal close packed (HCP) lattice structure has low ductility in comparison to conventional metallic materials having a body centered cubic (BCC) or face centered cubic (FCC) lattice structure, they are generally classified into a difficult processing material having low plastic workability. Thus, industrially used magnesium alloys are being used in the form of a cast instead of a forge.
As in common metals, a grain refiner added in a casting process of a magnesium alloy may provide various advantages such as an improvement in mechanical properties, a decrease in casting defects, suppression of segregation, an improvement in formability, and an improvement in surface properties.
For example, like an AZ-series magnesium alloy, as a magnesium (Mg)-aluminum (Al)-zinc (Zn)-based alloy having excellent corrosion resistance, and an AM-series magnesium alloy, as a magnesium (Mg)-aluminum (Al)-based alloy having excellent ductility, most of commercial magnesium alloys contains aluminum, wherein, as a grain refinement mechanism of the magnesium alloys containing aluminum, there currently are a heterogeneous nuclei theory and a carbon segregation theory.
Among these theories, the heterogeneous nuclei theory is a theory in which aluminum and carbon in a melt are combined to form a carbide by adding various inorganic compounds or gas containing carbon to the melt and particles of the carbide act as nucleation particles in a magnesium matrix during solidification of the melt to refine grains. Also, the carbon segregation theory is a theory in which a carbon element added to a melt inhibits grain growth by being segregated at a solid-liquid interface as initially solidified grains grow and thus, grains are refined.
As a typical method of refining a magnesium alloy based on the above theories, various methods, such as a superheating method in which a melt is superheated above a predetermined temperature, cooled to an injection temperature, and then injected, the Elfinal process in which ferric chloride (FeCl3) is added to a melt, a zirconium addition method in which zirconium (Zr) is added to a melt, and a carbon addition method, and refiners suitable for the methods have been developed.
The superheating method is a process in which a melt prepared by melting a magnesium alloy is superheated to a temperature of 180° C. to 300° C. or more, rapidly cooled to a casting temperature, and then injected, wherein there are limitations in that equipment costs and manufacturing costs are increased due to heat and rapid cooling processes, energy efficiency is reduced, and it is difficult to apply the method to large casting and continuous casting process.
The Elfinal process was developed in Germany in 1942 and is a method of refining grains by adding ferric chloride (FeCl3) to a melt near 740° C. to 780° C., but the process is disadvantageous in that, since iron (Fe) is added to an alloy, corrosion resistance of the alloy is reduced and chlorine gas harmful to the human body is generated.
The zirconium addition method, as a method of refining magnesium grains by adding 0.5 wt % to 1.0 wt % of zirconium, is currently widely used, but since a refinement effect disappears in a magnesium alloy containing aluminum and manganese alloying elements due to a reaction with these elements, the method may be difficult to be used, and the method may be difficult to be commercialized because a commercial magnesium alloy contains large amounts of these elements.
The carbon addition method is divided into a method of directly adding fine carbon powder to a melt and a method of adding an inorganic compound containing carbon. With respect to the carbon addition method, since there is no need to increase the temperature of the melt to a high temperature in comparison to the superheating method and the method is good in terms of economy, it is known as the most important refinement method for magnesium (Mg)-aluminum (Al)-based alloys.
However, the method of directly adding carbon powder in the above-described carbon addition method is a method of directly adding carbon black or fine carbon powder containing carbon to the melt, wherein since the carbon powder is not uniformly dispersed during the addition and most of the carbon powder may float on the melt to reduce refining efficiency, the method of adding the inorganic compound to the melt is more widely used.
As related art relating to a grain refiner of a magnesium alloy, Korean Patent No. 0836599 discloses a grain refiner of a magnesium alloy casting material and a refinement method. Specifically, the grain refinement method of a magnesium alloy casting material, which includes a refiner addition process, in which an aluminum-containing magnesium alloy is melted and magnesium carbonate (MgCO3) powder is then added in an amount of 0.5 wt % to 5.0 wt % based on an amount of the melt at a refiner addition temperature of 650° C. to 760° C., and a casting process, in which the melt is maintained for 5 minutes or more after the refiner addition process and is then cast, is disclosed. However, in a case in which the magnesium carbonate powder is added to the molten magnesium according to the refinement method, since the highly reactive magnesium carbonate is used in the form of powder having a high surface area, a reaction may vigorously proceed, the refiner may not be uniformly mixed in a lower portion of the melt, and the vigorous reaction may also cause a problem such as explosion.
Also, Korean Patent Application Laid-Open Publication No. 2009-0036239 discloses a grain refinement method of a magnesium alloy, and, specifically, the grain refinement method of a magnesium alloy, which includes the steps of preparing a molten magnesium alloy by melting a magnesium alloy using an electric furnace in an argon atmosphere, adding hexachloroethane (C2Cl6) to the molten magnesium alloy at a temperature of 780° C., and maintaining a mixed melt of the magnesium alloy and the hexachloroethane for 20 minutes to completely decompose the hexachloroethane, is disclosed.
However, in a case in which the hexachloroethane is added to the molten magnesium alloy according to the refinement method, fine grains may be obtained, but, when the hexachloroethane is added to the melt, a large amount of chlorine gas, which is fatal to the human body and corrodes metallic materials, may be generated.
Furthermore, Korean Patent No. 1214939 discloses a method of manufacturing a magnesium alloy, and, specifically, the method of manufacturing a magnesium alloy, which includes the steps of preparing a molten magnesium alloy by applying a protective gas to a magnesium alloy and heating the magnesium alloy to a melting temperature of the magnesium alloy to melt the magnesium alloy, adding a magnesium alloy grain refiner in the form of powder, pellets, rods, or wires to the molten magnesium alloy, and casting the molten magnesium alloy to form a fine-grained magnesium alloy casting material, is disclosed.
However, in a case in which the powder is directly added according to the method of manufacturing a magnesium alloy, a lower yield may be obtained in comparison to the amount of the added powder due to a phenomenon, in which the powder is discharged onto a surface or floats to the surface of the melt, and, when the refiner is added in the form of the pellets, the pellets are not completely decomposed, but partially compressed inclusion agglomerates may remain in the melt. Also, since manganese carbonate, as a carbonate-based material, is highly reactive regardless of the shape of the refiner, the melt boils due to carbon dioxide gas generated during decomposition when the manganese carbonate is directly added to the molten magnesium, and thus, there may be a risk of fire and a risk of oxidation of the surface of the molten magnesium.